This application is based on French Patent Application No. 01 05 032 filed Apr. 12, 2001, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to a method of stripping an optical fiber consisting of removing the coating from a particular area of the fiber.
2. Description of the Prior Art
An optical fiber consists of a core surrounded by a cladding layer delimiting an optical waveguide surrounded by a coating layer that protects the optical fiber as a whole. As a general rule the core and the cladding of the optical fiber are made of silica and the coating layer is made of polymers.
Optical fibers have many and varied applications. In some applications it is necessary to manipulate the silica optical waveguide, for example for optically writing a Bragg grating. To carry out this kind of operation, the fiber must be stripped, etched and then recoated. It may also be necessary to strip the optical fiber at one end, for example to execute a weld or a connection. The method according to the invention can be used to strip any portion of the optical fiber, whether in the middle or at an end. By stripping is meant removing the protective polymer from the optical waveguide consisting of the core and the cladding, that is to say, in the remainder of the text, removing the coating in order to bare the silica.
The present invention relates essentially to a method of stripping an optical fiber in order to bare the silica.
The person skilled in the art knows various stripping methods. Stripping methods are subject to heavy constraints, however. This is because the length of the stripped fiber portion must be carefully controlled, the high strength of the fiber must not be compromised, and the edges of the coating at the limits of the stripped portion must be clean.
What is more, the stripped fiber becomes extremely fragile and delicate to manipulate.
One method routinely used by persons skilled in the art is described in the paper: xe2x80x9cEffect of chemical stripping on the strength and surface morphology of fused silica optical fiberxe2x80x9d. Proc. Soc. Photo Optique Instrumentation Eng., vol 2074 pp 52-58. This kind of method consists of plunging the fiber portion to be stripped for 20 seconds into a bath of 98% sulfuric acid heated to 180-200xc2x0 C. To do this, it is necessary to coil the fiber. The stripped fiber is then cleaned with ethanol and/or acetone.
The above prior art method has many drawbacks. Coiling the fiber immersed in the acid bath increases the risk of impinging on the silica, which is bared after the polymer of the coating is degraded, which could reduce the mechanical strength of the stripped fiber. It is also difficult to control the length of the stripped portion because the sulfuric acid migrates rapidly into the coating by capillary action. This causes an uneven edge of the remaining coating (known to the person skilled in the art as a xe2x80x9ccauliflower edgexe2x80x9d).
The above method is also hazardous because of the proximity of inflammable products, such as alcohol, and pure sulfuric acid. The risk of acid burns is not negligible. All of this leads to heavy industrial constraints associated with temperature, relative humidity and, of course, rules for manipulation of the products.
Other prior art methods are based on the principle of softening the coating of the fiber by heating it, and then scraping it off.
For example, the international patent application WO 00/02076 proposes a method of stripping the end of an optical fiber by heating the fiber and scraping off the coating when it has become sufficiently softened.
Similarly, U.S. Pat. No. 6,123,801 describes a method of stripping any portion of an optical fiber, including a portion between its ends, by using a hot jet of inert gas to soften the coating. The power of the jet is such that the coating is blown off the fiber when it has become sufficiently softened. The jet is moved along the whole length of the portion of fiber to be stripped.
These prior art methods have all or some of the drawbacks already cited, in particular the use of heat.
Furthermore, it is noteworthy that none of the above prior art methods has yet been industrialized on a large scale.
The object of the present invention is to eliminate the drawbacks of the prior art.
To this end, the invention proposes a two-stage method, a first or mechanical stage which makes one or more cuts in the coating of the fiber and a second or chemical stage which softens the coating by the action of a stripping gel. The method according to the invention produces a stripped portion with clean edges and of perfectly controlled length, without compromising the mechanical strength of the optical fiber.
The invention relates more specifically to a method of stripping an optical fiber including a silica optical waveguide and a coating to be removed over a portion to be stripped, the method including the following steps:
making a cut in the coating of the portion to be stripped,
depositing a stripping gel on the coating of the portion to be stripped, and
cleaning the portion to be stripped to remove the coating and expose the silica of the fiber.
According to one feature of the invention the cut has a depth controlled in accordance with the nature of the fiber to be stripped in order to protect the optical waveguide of the fiber.
According to another feature of the invention the cut is circular.
In one application of the invention at least two cuts are made in the coating of the portion of the fiber to be stripped and the spacing between the cuts is adapted to suit the length of the portion to be stripped.
According to one feature of the invention the portion to be stripped is placed in a receptacle adapted to receive the stripping gel.
According to one feature of the invention the stripping gel has a suitable viscosity from 3 000 cps to 5 000 cps.
According to one feature of the invention the portion to be stripped is cleaned by cleaning jets adapted to clean the surface of the silica and to make a clean break in the coating at the edge of the portion to be stripped.
According to one feature of the invention the optical fiber is placed in a cassette with the portion to be stripped under tension.
According to one feature of the invention the portion of the optical fiber to be stripped is under a controlled tension and the cassette includes indexing means.
According to one advantageous feature of the invention the optical fiber remains in the cassette throughout the stripping process.
In respective embodiments of the invention the portion to be stripped is in the middle of the fiber and at an end of the fiber.
The present invention further relates to a system for implementing a method in accordance with the invention, which system includes:
a station for making a cut in the coating of the portion of the fiber to be stripped, and
a stripping station for effecting the following operations:
positioning the portion of the fiber to be stripped in a receptacle,
depositing a stripping gel in the receptacle and allowing it to act on the coating of the fiber, and
cleaning and drying the stripped fiber portion.
According to one feature of the invention the system for implementing the method further includes a cassette for supporting the optical fiber and maintaining the portion of the fiber to be stripped under tension.
The present invention also provides an optical fiber comprising a silica waveguide and a coating that is removed over a stripped portion, in which fiber the edges of the coating at the ends of the stripped portion are smooth and oblique with a longitudinal depth less than twice the diameter of the optical fiber.
The method according to the invention preserves the specifications of the fiber defined in terms of the constraints applying to its mechanical strength, a controlled and reproducible length of the stripped portion, and the quality of the edges of the coating.
Furthermore, the method according to the invention can be carried out at room temperature and without manipulating unstable hazardous products. The stripping gel is stable at room temperature and normal relative humidity.
What is more, the method according to the invention lends itself readily to large-scale industrialization.
Other features and advantages of the invention will become clearly apparent after reading the following description, which is given by way of illustrative and non-limiting example and with reference to the accompanying drawings.